Irregularities Observed at the Edge of a Mid‐Latitude Ionospheric Depletion Following a Geomagnetic Storm.

This manuscript presents the analysis of data from multiple ground‐ and space‐based sensors in the North American region before, during, and after the 12 October 2021 geomagnetic storm. Total electron content (TEC) and electron density data show the formation and equatorward propagation of a mid‐latitude trough at ∼50°N followed by the appearance of a wider depletion region (∼15° in latitude) at lower latitudes. During the recovery phase on the 13th, the equatorward edge of this depletion region settled at around 30° latitude and exhibited a steep density gradient. By the 14th, this sharp boundary had disappeared. Near this edge on the 13th, small‐scale irregularities formed. The impact of these was observed within Global Positioning System data as elevated rate of TEC index (ROTI) and presented as strong 35 MHz scintillations of cosmic radio sources as well as spread‐F within ionograms from multiple digisonde systems. GPS and 35‐MHz data demonstrated that the irregularity region was narrowly confined (≲5° wide) near the edge of the depletion region. The 35‐MHz scintillation data also showed that the irregularities were moving relatively slowly at ∼7 m s−1, likely toward the southeast. Density and velocity measurements demonstrated that the conditions near the depletion boundary were highly favorable to the gradient drift instability (GDI) with the one‐dimensional growth rate estimated to be ∼0.01 s−1. Since these conditions persisted for many hours, this growth rate was more than sufficient for the GDI to be considered the primary driver of irregularity formation in this case. Plain Language Summary: Earth's ionosphere, the ionized portion of the upper atmosphere, is a dynamic environment, often beset with irregularities and disturbances that interfere with radio frequency signals that travel through it. In this regard, mid‐latitudes are usually quite tame relative to the equatorial and polar regions. However, when significant geomagnetic disturbances, or "storms," occur, this normally placid region can become anything but. This paper presents the results of a study that examines the impact of a storm over North America, using several space‐ and ground‐based sensors. Of particular interest is the formation of a density depletion region during the recovery period following the onset of the storm. Multiple sensors show evidence of this depletion and still others show a preponderance of small‐scale irregularities (as small as a few kilometers) near its southern boundary. A holistic analysis indicates that these irregularities are consistent with a turbulent cascade moving relatively slowly, likely toward the south or southeast. Conditions near the depletion edge were observed to be conducive with the so‐called gradient drift instability. This is known to be a primary driver of irregularity formation within other settings, and appears to be the culprit in this case as well. Key Points: A multitude of ground‐ and space‐based sensors were used to study a storm‐induced mid‐latitude ionospheric depletion over North AmericaIrregularities at the equatorward edge of the depletion at ∼30°N were detected with radio frequency systems from a few MHz to 1.5 GHzSpace‐based in situ and remote sensing data point to the gradient drift instability as the most likely driver of irregularity formation [ABSTRACT FROM AUTHOR]